1. Field of the Invention
The present invention relates to a method and apparatus for detecting an abnormal state of an evaporative emission-control system having a canister containing an adsorbent and disposed between a fuel tank and an intake air passage.
2. Description of the Related Art
Generally, modern automobiles are equipped with an evaporative emission-control system having a canister filled with an adsorbent such as an activated charcoal, for trapping fuel vapor (HC) from a fuel tank and preventing an escape thereof to the open air. Fuel-vapor is caused by evaporation, and a large part of the atmosphere in the fuel tank is composed of fuel-vapor. In the evaporative emission-control system, fuel-vapor from the fuel tank flows to the charcoal canister, the charcoal particles adsorb and retain the fuel-vapor, and when the engine is run and a negative pressure is generated downstream of a throttle valve, air flows through the charcoal canister on the way to the intake air system, e.g., intake air pipe, due to the negative pressure. This intake air picks up the fuel-vapor trapped in the canister and carries it to the intake air pipe, where it is mixed with the air-fuel mixture, fed to the engine and burned, instead of being allowed to enter the atmosphere as fuel-vapor.
In this evaporative emission-control system, when a purge operation is carried out, i.e., when the fuel-vapor trapped in the canister is removed by the air drawn in by the intake-manifold vacuum, and a vapor-laden air (a purged gas) is mixed with the air-fuel mixture, an air-fuel ratio of the engine is changed in accordance with the purged gas density. Accordingly, it is necessary for the evaporative emission-control system to adjust an air-fuel ratio correction coefficient FAF in accordance with the amount of fuel-vapor purged from the canister when the purge operation is carried out. An air-fuel ratio correcting apparatus for the evaporative emission-control system is, for example, disclosed in Japanese Unexamined Patent publication No. 63-186955, wherein an an amount of fuel injection is corrected in accordance with an amount of fuel-vapor assumed by calculating a center value of the control of the air-fuel ratio correction coefficient FAF at an idling state and at a light load state.
If the amount of fuel-vapor purged from the canister of the evaporative emission-control system is not accurately calculated, the air-fuel ratio feedback control of the engine is not normally operated and the emission characteristics and the fuel consumption of the vehicle will be worsened.
As a countermeasure to cope with an above-described problem in the air-fuel feedback control system, an apparatus for detecting an abnormal state of the evaporative emission-control system has been proposed. For example, an apparatus which diagnoses whether or not the evaporative emission-control system is operating correctly by examining a change in the air-fuel ratio between an ON and OFF of the execution of the purge operation while the engine is running, has been proposed.
In this abnormal state detecting apparatus, a diagnosis of the evaporative emission-control system is carried out by detecting whether or not the air-fuel ratio becomes rich when the purge operation is executed, since if the evaporative emission-control system is normal and the fuel-vapor from the fuel tank is properly trapped in the canister, the air-fuel ratio becomes rich when the purge operation is executed. Namely, an incorrect operation of the evaporative emission-control system is detected by the abnormal state detecting apparatus when the change of the air-fuel ratio between an ON and OFF of the execution of the purge operation is smaller than a predetermined value.
Nevertheless, in the above-described abnormal state detecting apparatus, the evaporative emission-control system is erroneously detected to be abnormal when the fuel-vapor is not properly trapped in the canister or the concentration of the purged gas from the canister is equal to the stoichiometric air-fuel ratio.
Further, there has been proposed another type of abnormal state detecting apparatus having a sensor such as a pressure sensor or an air-flow sensor for detecting a flow of the vapor-laden air in a purge pipe connecting the canister and the air intake passage, and determining an abnormal state of the evaporative emission-control system in accordance with the output signal of the sensor when the purge operation is executed.
This type of the abnormal state detecting apparatus, however detects only the amount of air flow in the purge pipe and cannot detect whether or not the air flowing in the purge pipe includes fuel-vapor. Namely, if the fuel-vapor from the fuel tank cannot be trapped in the canister because of a malfunction thereof, the above-described apparatus detects that the purge system is normal and the incorrect operation of the canister cannot be detected.